A combined biochemical and anatomical study is underway to identify neurotransmitters of retinal neurons and in neurons of visual pathways, to examine the development of neurotransmitter systems in retina, and to det ermine the role of chemical neurotransmitters in the processing of visual information. Two systems are being investigated: (1) the simple visual system of the horsehoe crab Limulus polyphemus, and (2) developing mammalian retinal neurons. (1) Studies of neurotransmitters in the Limulus visual system revealed that centrifugal fibers which project from the brain to the retina and innervate photoreceptor cells synthesize, store, and release the biogenic amine octopamine and amine conjugates. Photoreceptors respond to octopamine by increasing intracellular cAMP, and octopamine causes the phosphorylation of at least two photoreceptor phosphoproteins. These studies show that the central nervous system can modulate the biochemistry of photoreceptor cells via centrifugal fibers. Some of the ciochemical changes induced by centrifugal innervation may influence basic processes of photoreceptor cell function such as phototransduction, adaptation or membrane turnover. (2) The development of synaptic function of mammaliam retinal neurons is being studied in normal intact rat retinas as well as in monolayer cultures prepared from embryonic rat retinas. The histogenesis of different biochemical classes of amacrine cells is beng examined as is the development of the ability of these neurons to synthesize and store neurotransmitter substances. The investigation of effects of depolarization on GABAergic function of retinal neurons in monolayer culture has continued using assays of glutamic acid decarboxylase (GAD) activity, localization of GAD-like immunoreactivity and GABA uptake autoradiography.